Radio frequency identification (“RFID”) is becoming increasingly common. RFID uses wireless radio communications to identify and collect information about objects. Information about an object may be created or updated “on the fly” after the object is identified. RFID may be used in a wide range of applications including identity cards.
RFID systems consist of one or more readers and transponders, contactless memory tags that communicate with the reader. Transponders may be carried on identity cards. The transponders contain data allowing the card to be identified and are made of a memory associated with a radio frequency communication block. The reader is connected to an antenna which transmits and receives the radio frequency signal to and from the transponders. In FIG. 1, a reader 68 of the prior art may communicate with all the cards 70, 72, 74, and 76 in its RF field 66. The reader 68 cannot communicate with a card 78 outside its field 66.
When a reader communicates with the transponders or cards within its fields, collisions occasionally occur if more than one transponder or card tries to reply to the reader at the same time. When a collision occurs, the reader does not receive a clear response from either card.
Anticollision mechanisms may be employed to prevent more than one card from transmitting at the same time. A probabilistic anticollision mechanism repeatedly prompts cards to reply within a single timeslot. However, using this approach, the presence of multiple cards may not be detected until the responding card is removed from the field or the state where it can respond to requests from the reader. A timeslot anticollision method may be employed where the reader will send out polling commands prompting responses from cards which have chosen a timeslot in which to respond.
It would be desirable to have a method which could build a list of cards in the reader's field to know which cards are present before processing a card. It would be further desireable to know which state the cards are in and to be able to change the card's state.